Preview: rename automatically locks PDFs

Why do my PDFs lock automatically in Preview when I edit the file name?

Here is the process I undertake with PDFs on a regular basis that results in an unintended and unusual file locking behaviour:

1) Navigate to any ordinary PDF using Safari. Here's one: https://images.apple.com/ca/promo/pdf/MobileMe_09_CE.pdf

('Ordinary' means 'not protected')

2) Hover near the bottom of the Safari window to bring up the little tool palette. The third icon from the left is for the tool "Open in Preview". Click it.

3) In preview, click on the file name at the top of the file window. In the box that drops down, select the file name again, highlight it, and replace it with a new name (Newfile will do nicely). Change the file location if you wish (beside "Where:") or leave it where it is. I generally send my PDFs to "Downloads"

4) Press return

Watch the top of the file window. After a brief pause, the file name will change, and then the word 'Locked' will appear.

Interesting, the locking of the file, though it is unintended and occurs without 'requesting' the lock by clicking the "Locked" check box, is reversible. If you once again click on the file name to drop down the "Name, Tags, Where" editing box, you can uncheck 'Locked', and the file happily returns to the unlocked state.

How can I stop this strange and unintended behaviour?